McLean Ser. No. 766,648 describes a windmill having a vertical axis and means for protecting the airfoils from damage during severe wind storms.
Shaffer U.S. Pat. No. 1,003,661 positions a wind turbine near the ground, and provides a collapsable canvas tube which can be raised to a height for capturing higher level winds for actuating the turbine.
Savonius U.S. Pat. No. 1,697,574 describes advantageous shapes for a pair of airfoils for a windmill.
Magoveny et al U.S. Pat. No. 3,938,907 describes vanes which are attended and withdrawn in response to the speed of the turbine.
Sarchet U.S. Pat. No. 4,066,911 shows a windmill driving a generator having a feedback system so that the tautness of the sails of the horizontally rotating airfoils is responsive to the generated voltage.
An article about "Windmills for Rural Use" by S. P. G. Raju et al presented at the Intl. Solar Energy Congress, held at New Delhi, Jan. 16-21, 1978 describes a vertical axis windmill having a pair of Savonius airfoils.
Although technologists have been seeking to build better windmills for many centuries, there has been an unsatisfied demand for apparatus which could be predominantly assembled in a factory, which could be suitable for a variety of locations of varying wind conditions, and which could safely withstand the stresses of windstorms.
My invention relates to various types of wind turbines such as those classified on pages 18 and 19 in the publication entitled Wind Machines, National Science Foundation, Superintendent of documents Stock No. 038-000-00272-4. Wind turbines sweeping a large enough area to yield substantial power during light winds are vulnerable to damage by wind loading of their components and support structures during high winds.
In the prior art, varieties of arrangements have been used to protect wind machines from damage during high winds, and the machine elements and their supports have been made large and strong in order to sustain the stresses due to wind loads and vibrations due to turbulence. Such structures are heavy and costly. They are subject to friction losses in their bearings due to the extra weight needed for reliability. This friction stops power output during light winds.
In the prior art wind turbine rotors have been elevated on towers so as to intercept faster winds. Equipment so elevated necessitates climbing up the tower or other support structure in order to gain access to the working parts. This is especially disadvantageous during inclement weather, when it might be necessary to climb to the working mechanism to secure some failed part against catastrophic damage.
In the prior art static transducers which convert wind energy directly into magnetic, electric, or thermal energy also must be exposed to the violence of various wind conditions; and so they, too are subject to damage in wind gusts. The cost of mounting such static transducers is high due to the extra material and additional strengthening members needed.
One kind of wind machine whose advantages have been devalued because of the above-described considerations is the kind identified in the above-referenced page 19 as the Split Savonius wind machine. A rotor of this shape was disclosed in U.S. Pat. No. 1,200,308 issued in 1916 to J. C. Bunnell under the title Water Motor. The Savonius rotor is disclosed in U.S. Pat. No. 1,697,574 issued in 1929 to S. J. Savonius under the title: Rotor Adapted To Be Driven By Wind Or Flowing Water. Numerous reports or practical applications, experimental studies, and theoretical analyses are referenced in Wind Tunnel Performance Data For Two-And Three-Bucket Savonius Rotors, Sandia Laboratories Energy Report No. SAND76-0131, printed July 1977. Still other publications describe the constructions of Savonius split rotors by means of fabric and supporting wires so that the blades of the rotor are actually sails stiffened with taut wires. One such paper was presented at the Solar Energy Congress in New Delhi, India in January 1978, and was titled: Windmills For Rural Use. The Authors were Professor S. P. Govinda Raju and Professor R. Narasimha, Indian Institute of Science, Bangalore-12, India. The design reported in this paper describes joints in the tension wires so constructed as to yield and slacken the sails during high winds, and thus unload the structure. This and other developments have improved the ability of Split Savonius rotors to withstand high winds without excessive costs, but it has nonetheless been concluded by some skilled in the art that this kind of wind energy conversion system is not worth developing commercially.
However, the development of the split Savonious continues. In the above-cited report SAN76-0131, we find in the introduction the statement: ". . . because the technology required to fabricate a Savonious rotor is considerably less than that required for more sophisticated types of wind turbines, Savonius rotors may find more applications in developing countries as well as in do-it-yourself projects."
In designing wind energy conversion systems to convert the most energy per unit expense, not only is it necessary to intercept a large cross section of the wind, but it is also desirable to intercept the wind at higher elevations above ground. The publication Weatherwise, Vol. 27, No. 6, Dec. 1974, page 239, FIG. 4, portrays the increase of the power in the wind at heights. Much more power is gained from a given wind by additional elevation of the airfoils intercepting the wind.